1. Field
The present invention relates generally to communication systems, and more specifically to a method and an apparatus for estimating a reverse link maximum data rate and for estimating power required for transmission of data at a rate of data in a communication system.
2. Background
Communication systems have been developed to allow transmission of information signals from an origination station to a physically distinct destination station. In transmitting an information signal from the origination station over a communication channel, the information signal is first converted into a form suitable for efficient transmission over the communication channel. Conversion, or modulation, of the information signal involves varying a parameter of a carrier wave in accordance with the information signal in such a way that the spectrum of the resulting modulated carrier wave is confined within the communication channel bandwidth. At the destination station the original information signal is reconstructed from the modulated carrier wave received over the communication channel. In general, such a reconstruction is achieved by using an inverse of the modulation process employed by the origination station.
Modulation also facilitates multiple access, i.e., simultaneous transmission and/or reception, of several signals over a common communication channel. Multiple access communication systems often include a plurality of remote subscriber units requiring intermittent service of relatively short duration rather than continuous access to the common communication channel. Several multiple-access techniques are known in the art, such as Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA). Another type of multiple access technique is a Code Division Multiple Access (CDMA) spread spectrum system that conforms to the “TIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wide-Band Spread Spectrum Cellular System,” hereinafter referred to as the IS-95 standard. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE-ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” and U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,” both assigned to the assignee of the present invention.
A multiple access communication system may be a wireless or wire-line and may carry voice and/or data. An example of a communication system carrying both voice and data is a system in accordance with the IS-95 standard, which specifies transmitting voice and data over the communication channel. A method for transmitting data in code channel frames of fixed size is described in detail in U.S. Pat. No. 5,504,773, entitled “METHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION”, assigned to the assignee of the present invention. In accordance with the IS-95 standard, the data or voice is partitioned into code channel frames that are 20 milliseconds wide with data rates as high as 14.4 kbps. Additional examples of a communication systems carrying both voice and data comprise communication systems conforming to the “3rd Generation Partnership Project” (3GPP), embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), or “TR-45.5 Physical Layer Standard for cdma2000 Spread Spectrum Systems” (the IS-2000 standard).
In a multiple access communication system, communications between users are conducted through one or more base stations. A first user on one subscriber station communicates to a second user on a second subscriber station by transmitting data on a reverse link to a base station. The base station receives the data and can route the data to another base station. The data is transmitted on a forward link of the same base station, or the other base station, to the second subscriber station. The forward link refers to transmission from a base station to a subscriber station and the reverse link refers to transmission from a subscriber station to a base station. Likewise, the communication can be conducted between a first user on one mobile subscriber station and a second user on a landline station. A base station receives the data from the user on a reverse link, and routes the data through a Public Switched Telephone Network (PSTN) to the second user. In many communication systems, e.g., IS-95, W-CDMA, IS-2000, the forward link and the reverse link are allocated separate frequencies.
An example of a data only communication system is a High Data Rate (HDR) communication system that conforms to the TIA/EIA/IS-856 industry standard, hereinafter referred to as the IS-856 standard. This HDR system is based on a communication system disclosed in co-pending application Ser. No. 08/963,386, entitled “METHOD AND APPARATUS FOR HIGH RATE PACKET DATA TRANSMISSION,” filed Nov. 3, 1997, now U.S. Pat. No. 6,574,211, issued Jun. 3, 2003 to Padovani et al., assigned to the assignee of the present invention. The HDR communication system defines a set of data rates, ranging from 38.4 kbps to 2.4 Mbps, at which an access point (AP) may send data to a subscriber station (access terminal, AT). Because the AP is analogous to a base station, the terminology with respect to cells and sectors is the same as with respect to voice systems.
In a wireless communication system, maximizing a capacity of the communication system in terms of the number of simultaneous telephone calls that can be handled is extremely important. The capacity in a spread spectrum communication system can be maximized if the transmission power of each subscriber station is controlled such that each transmitted signal arrives at a base station receiver at the same signal level. However, if a signal transmitted by a subscriber station arrives at the base station receiver at a power level that is too low, quality communications cannot be achieved due to interference from the other subscriber stations. On the other hand, if the subscriber station transmitted signal is at a power level that is too high when received at the base station, communication with this particular subscriber station is acceptable but this high power signal acts as interference to other subscriber stations. This interference may adversely affect communications with other subscriber stations. Therefore, each subscriber station needs to transmit the minimum signal level expressed as e.g., a signal-to-noise ratio, that allows transmitted data recovery.
Consequently, the transmission power of each subscriber station within the coverage area of a base station is controlled by the base station to produce the same nominal received signal power or a signal to noise ratio at the base station. In an ideal case, the total signal power received at the base station is equal to the nominal power received from each subscriber station multiplied by the number of subscriber stations transmitting within the coverage area of the base station plus the power received at the base station from subscriber stations in the coverage area of neighboring base stations.
The path loss in the radio channel can be characterized by two separate phenomena: average path loss and fading. The forward link, from the base station to the subscriber station, operates on a different frequency than the reverse link, from the subscriber station to the base station. However, because the forward link and reverse link frequencies are within the same general frequency band, a significant correlation between the average path losses of the two links exists. On the other hand, fading is an independent phenomenon for the forward link and reverse link and varies as a function of time.
In an exemplary CDMA system, each subscriber station estimates the path loss of the forward link based on the total power at the input to the subscriber station. The total power is the sum of the power from all base stations operating on the same frequency assignment as perceived by the subscriber station. From the estimate of the average forward link path loss, the subscriber station sets the transmit level of the reverse link signal. This type of an open loop control is advantageous when there is a correlation between a forward link and a reverse link. Should the reverse link channel for one subscriber station suddenly improve compared to the forward link channel for the same subscriber station due to independent fading of the two channels, the signal as received at the base station from this subscriber station would increase in power. This increase in power causes additional interference to all signals sharing the same frequency assignment. Thus a rapid response of the subscriber station transmit power to the sudden improvement in the channel would improve system performance. Therefore, it is necessary to have the base station continually contribute to the power control mechanism of the subscriber station. Such a power control mechanism relies on a feedback, also referred to as a closed loop.
Each base station with which the subscriber station is in communication measures the received signal strength from the subscriber station. The measured signal strength is compared to a desired signal strength level for that particular subscriber station. A power adjustment command is generated by each base station and sent to the subscriber station on the forward link. In response to the base station power adjustment command, the subscriber station increases or decreases the subscriber station transmit power by a predetermined amount. By this method, a rapid response to a change in the channel is effected and the average system performance is improved. Note that in a typical cellular system, the base stations are not intimately connected and each base station in the system is unaware of the power level at which the other base stations receive the subscriber station's signal.
When a subscriber station is in communication with more than one base station, power adjustment commands are provided from each base station. The subscriber station acts upon these multiple base station power adjustment commands to avoid transmit power levels that may adversely interfere with other subscriber station communications and yet provide sufficient power to support communication from the subscriber station to at least one of the base stations. This power control mechanism is accomplished by having the subscriber station increase its transmit signal level only if every base station with which the subscriber station is in communication requests an increase in power level. The subscriber station decreases its transmit signal level if any base station with which the subscriber station is in communication requests that the power be decreased. A system for base station and subscriber station power control is disclosed in U.S. Pat. No. 5,056,109 entitled “METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM,” issued Oct. 8, 1991, assigned to the Assignee of the present invention.
There is a relationship between a transmission power and a rate of data to be transmitted. Communication systems, in general, do not allow an instantaneous change of rate of data. If a transmission channel link condition changes, resulting in a need to change a transmission power and a data rate during the interval when a rate of data cannot be changed, the transmitted data may be erased. Therefore, there is a need in the art to estimate a rate of data that can be transmitted without an erasure under all channel conditions, or alternatively to estimate power required for transmission of data at a rate of data.